Human complement factor I: analysis of cDNA-derived primary structure and assignment of its gene to chromosome 4

Human complement factor I: its expression by insect cells and its biochemical and structural characterisation

Factor I is a five-domain plasma serine protease which is essential for the regulation of the complement system. In order to express this, the factor I coding sequence was cloned into a recombinant baculovirus system, which was used to infect Trichoplusia ni cells. Using the native factor I leader sequence, recombinant factor I (rFI) was secreted into the culture medium. Purified rFI was recognised by polyclonal antisera and by the factor I-specific monoclonal antibody MRC-OX21. SDS PAGE showed that rFI was processed into two chains with molecular weights of 48,000 and 36,000. Amino acid sequence analysis showed that the N-terminal sequences of the rFI chains were the same as those of serum-derived factor I (sFI), confirming that processing was correct. Since both molecular weights were less than those observed for sFI, this is attributed to the replacement of complex-type oligosaccharides by high mannose ones in rFI. C3(NH,) cleavage assays showed that rFI had 55% the activity of sFI. Circular dichroism and Fourier transform infrared spectroscopy showed that the protein folding of rFI and sFI were very similar. Both had a secondary structure low in alpha-helix and high in beta-sheet, as expected from crystal structure and multiple sequence alignment analyses. It is inferred that the reduced activity of rFI is attributable to its changed glycosylation. The availability of rFI and structures for the domains in factor I makes possible new approaches to determine the molecular basis of its interactions with factor H and C3b.

Cloning and characterization of the promoter for the human complement factor I (C3b/C4b inactivator) gene

Complement factor I is a serine proteinase that regulates the classical and alternative pathways of complement by cleaving C3b and C4b and preventing the assembly of C3 and C5 convertase enzymes. In order to understand the regulation of factor I gene expression in liver cells, 4kb of the 5' flanking region of the gene was cloned, and the 1474-bp 3'-end was sequenced and shown to contain a number of transcription factor consensus sequences. A major and two minor transcription start sites were identified, respectively, at 152, 178, and 198bp upstream of the translation start site by primer extension analysis. The transcriptional activity of the 1474-bp fragment was analyzed by fusion of 5' deletion constructs to a cat-encoding gene expression vector and transient transfections into Hep G2 cells. A 273-bp fragment located at -112 to +161 relative to the major transcription start site was sufficient for promoter activity. The 3' fragment spanning +3 to +161 and containing a TATA-like element did not demonstrate promoter activity, suggesting that the core promoter resides in a 115-bp sequence located between -112 and +3. This region contains an Inr-like element overlapping the major cap site and a CTF-NF1 element, two potential CCAAT boxes and an AP-2 element partially overlapping an Sp-1 site. Thus, factor I promoter may belong to the TATA-less Inr-driven class II promoters whose transcription is regulated by Sp-1. The transcriptional activity of the 1474-bp 5' flanking fragment was upregulated by PMA, IL-6 and TNF-alpha, suggesting that factor I may be an acute phase reactant.

Molecular cloning of a novel brain-specific serine protease with a kringle-like structure and three scavenger receptor cysteine-rich motifs

In order to find serine proteases specifically expressed in brain, we designed degenerate mixed primers for consensus sequences of serine protease domains. By PCR utilizing the primers, we have cloned a novel sequence from reverse transcripts of total RNA of mouse brain and used it as a probe to screen a mouse brain cDNA library. Overlapping cDNAs encoding a precursor of a novel brain specific serine protease (BSSP-3) were cloned. DNA insert of the longest clone consisted of 2614-bp with an entire open reading frame encoding a secretory/membrane-anchored precursor protein consisting of 761 amino acids (AA) which may be processed to yield an active enzyme of 245 AA. As found in known serine proteases, BSSP-3 enzyme domain contained a catalytic triad which consists of AA residues essential for the enzyme activity. In the upstream region of the enzyme domain that resides at C-terminus of the precursor protein, there are, from N-terminus to downstream, a sequence similar to a kringle structure and three repetitive ones highly similar to the scavenger receptor cysteine-rich (SRCR) motifs. Northern blot analysis demonstrated that mBSSP-3 mRNA was specifically expressed in the mouse brain, lung and kidney. We concluded that a novel brain serine protease, BSSP-3, is a new member of kringle and SRCR superfamilies.

A novel member of an ancient superfamily: sponge (Geodia cydonium, Porifera) putative protein that features scavenger receptor cysteine-rich repeats

Proteins featuring scavenger receptor cysteine-rich (SRCR) domains are prominent receptors known from vertebrates and from one phylum of invertebrates, the echinoderms. In the present study we report the first putative SRCR protein from the marine sponge Geodia cydonium (Porifera), a member of the lowest phylum of contemporary Metazoans. Two forms of SRCR molecules were characterized, which apparently represent alternative splicing of the same transcript. The long putative SRCR protein, of 1536 aa, features twelve SRCR repeats, a C-terminal transmembrane domain and a cytoplasmic tail. The sequence of the short form is identical with the long form except that it lacks a coding region near the C terminus, thus the 1195 aa deduced protein consists of only the first ten SRCR domains and the last 26 C-terminal aa residues, without the transmembrane domain. Homology searches revealed that the sponge putative SRCR protein shares with bovine T-cell antigen WC1 29.2% identity in 1054 aa overlap, 33.9% identity in 475 aa overlap with sea urchin speract and 56% identity in 110 aa overlap with macrophage scavenger receptor type I. Based upon the number and location of the conserved Cys residues, the sponge SRCR domain repeats were classified as belonging to group A of the SRCR superfamily. With twelve SRCR repeats, one more than those in any of the previously described SRCR proteins, and several membrane-bound and soluble forms, it seems that the most primitive known member of this family may be the structurally most complex one among SRCR containing proteins.

Characterization of mouse CD6 with novel monoclonal antibodies which enhance the allogeneic mixed leukocyte reaction

Human CD6 is a cell surface protein expressed by thymocytes, mature T cells, a subset of B cells and certain cells of the brain. On human T cells, CD6 has been shown to act as a co-stimulatory molecule which modulates T cell receptor (TCR)-mediated T cell activation. To study further the recently identified mouse CD6 (mCD6), we generated and characterized a set of anti-mCD6 mAb. Anti-mCD6 mAb recognizing the mCD6 scavenger receptor cysteine-rich (SRCR) extracellular domains 1 and 3 were identified. mAb against SRCR domain 3, but not domain 1, inhibited the interaction of CD6 with a recently identified ligand, activated leukocyte cell adhesion molecule (ALCAM). Immunohistochemical analysis indicated that mCD6 expression was largely localized to the T cell areas of lymphoid tissue and, as previously reported in the human, CD6 was also expressed by neurons. CD6 was highly expressed on mouse T cells isolated from the spleen, lymph node and thymus as demonstrated by two-color immunofluorescence analysis. The CD4+ and CD8+ cells in these lymphoid compartments expressed similar levels of CD6. Immunoprecipitation studies showed that mouse thymocytes predominantly express a CD6 isoform of approximately 130 kDa, while splenocytes predominantly express a CD6 isoform of approximately 100 kDa. Anti-mCD6 mAb enhanced allogeneic mixed leukocyte reactions (MLR), indicating that CD6-ALCAM interactions may regulate the proliferative capacity of T cells.

cDNA cloning, sequencing and chromosomal assignment of the gene for mouse complement factor I (C3b/C4b inactivator): identification of a species specific divergent segment in factor I

Factor I is an essential regulatory serine proteinase of the complement cascade. It cleaves and inactivates the C3b and C4b constituents of the C3 and C5 convertases and thereby regulates many complement-mediated activities. The human protein is a heterodimer composed of a 50 kDa non-catalytic subunit (which contains several domains, i.e. FIM, CD5, LDLr type A) disulfide linked to a 38 kDa catalytic subunit. Recent characterization of Xenopus factor I cDNA revealed a 29 residue negatively charged region in its heavy chain which is absent in the human protein (Kunnath-Muglia et al., Molec. Immun. 30, 1249-1256, 1993). We report the complete cDNA sequence of mouse factor I as well as a partial chicken factor I cDNA sequence. Alignment of these two sequences with the published sequences for human and Xenopus proteins (a) demonstrates an overall conservation of primary structure and domain organization of mouse factor I, and (b) defines a divergent segment (D segment) in each species. In Xenopus protein, the D segment includes the 29 residue negatively charged region. In each of the four species examined, the D segment differed in length, sequence, organization, and number of repeated subregions. These differences reflect a considerable evolution of D segment. The significance of the diversity of the D segment is at present unclear. We also report the chromosomal localization of the mouse factor I gene (Cfi) to distal chromosome 3 near Egf.

Control of the complement system

The complement system has developed a remarkably simple but elegant manner of regulating itself. It has faced and successfully dealt with how to facilitate activation on a microbe while preventing the same on host tissue. It solved this problem primarily by creating a series of secreted and membrane-regulatory proteins that prevent two highly undesirable events: activation in the fluid phase (no target) and on host tissue (inappropriate target). Also, if not checked, even on an appropriate target, the system would go to exhaustion and have nothing left for the next microbe. Therefore, the complement enzymes have an intrinsic instability and the fluid-phase control proteins play a major role in limiting activation in time. The symmetry of the regulatory process between fluid phase and membrane inhibitors at the C4/C3 step of amplification and convertase formation as well as at the MAC steps are particularly striking features of the self/nonself discrimination system. The use of glycolipid anchored proteins on membranes to decay enzymes and block membrane insertion events is unlikely to be by chance. Finally, it is economical for the cofactor regulatory activity to produce derivatives of C3b that now specifically engage additional receptors. Likewise, C1-Inh leads to C1q remaining on the immune complex to interact with the C1q receptor. Thus the complement system is designed to allow rapid, efficient, unimpeded activation on an appropriate foreign target while regulatory proteins intervene to prevent three undesirable consequences of complement activation: excessive activation on a single target, fluid phase activation, and activation on self.

The membrane-proximal scavenger receptor cysteine-rich domain of CD6 contains the activated leukocyte cell adhesion molecule binding site

Binding studies with a CD6 immunoglobulin fusion protein (CD6 Rg) resulted in the identification and cloning of a CD6 ligand. This ligand was found to be a member of the immunoglobulin supergene family and was named ALCAM (activated leukocyte cell adhesion molecule). Cell adhesion assays showed that CD6-ALCAM interactions mediate thymocyte-thymic epithelium cell binding. ALCAM is also expressed by activated leukocytes and neurons and may be involved in interactions between T cells and activated leukocytes and between cells of the immune and nervous systems, respectively. Herein we describe the preparation of domain-specific murine CD6 Rg fusion proteins and show that the membrane-proximal SRCR (scavenger receptor cysteine-rich) domain of CD6 contains the ALCAM binding site. We also show that mAbs which bind to this domain preferentially block CD6-ALCAM binding. These results demonstrate that the membrane-proximal SRCR domain of CD6 is necessary for CD6 binding to ALCAM and provide the first direct evidence for the interaction of an SRCR domain with a ligand.

Processing of human factor I in COS-1 cells co-transfected with factor I and paired basic amino acid cleaving enzyme (PACE) cDNA

Factor I is an active serine proteinase in plasma that regulates both the classical and alternative complement pathways by cleaving C3b and C4b thereby preventing the assembly of C3 and C5 convertase enzymes. In this study, a full-length human factor I cDNA was cloned into the pMT2 expression vector and the pMT2-fI construct was expressed transiently in COS-1 cells and stably in CHO-K1 cells. The transfected COS-1 cells secreted large amounts of recombinant pro-factor I (85 kD). Co-transfection of COS-1 cells with pMT2-fI and the cDNA expression plasmid for PACE (paired basic amino acid cleaving enzyme), resulted predominantly in the secretion of a proteolytically processed form of recombinant factor I (heavy chain, 47 kD; light chain, 35 kD). Following co-transfection of pMT2-fI and pSVNeo.1 into CHO-K1 cells and selection in medium containing G418, a stably transfected clone was isolated that secreted pro-factor I (85 kd) and proteolytically processed factor I (heavy chain, 48 kD; light chain, 37 kD) in approximately equal amounts. The molecular sizes of the subunit chains of the expressed factor I were generally slightly smaller than those of human plasma factor I. The activity of recombinant factor I present in the culture supernatants of transfected COS-1 and CHO-K1 cells was assayed by its ability to cleave 125I-C3b in the presence of factor H and was found to be low when compared with factor I purified from human plasma. However, since the functional activity of purified factor I was reduced approximately 50% in the presence of conditioned medium from non-transfected cells, it is suggested that the cold C3b present in the factor I-deficient serum used to supplement the culture medium probably competed with the 125I-C3b tracer, thereby decreasing the sensitivity of the assay for the recombinant factor I proteins.

Seven loci on human chromosome 4 map onto sheep chromosome 6: a proposal to restore the original nomenclature of this sheep chromosome

Seven new loci, casein alpha-S1 (CSN1S1), casein alpha-S2 (CSN1S2), casein beta (CSN2), the Hardy-Zuckerman 4 feline sarcoma viral (v-kit) oncogene homolog (KIT), albumin (ALB), phosphodiesterase cyclic GMP (rod receptor) beta polypeptide (PDEB), and complement component 1 (IF), were assigned to sheep Chromosome (Chr) 6 by Southern hybridization to a panel of chromosomally characterized sheep x hamster cell hybrids. By isotopic in situ hybridization, CSN2 was regionally localized to sheep Chr (OOV) 6q22-q31, anchoring this syntenic group of markers on to OOV6 and confirming its homology at a molecular and cytological level with cattle Chr 6. The assignment of these loci, from PDEB (located on human Chr 4p16.3) to IF (on HSA4q24-q25), and the observation that interleukin 2 (IL2, on HSA4q26-q27) and tryptophan 2,3-dioxygenase (TDO2, on HSA4q31) are not located on OOV6, is further evidence of the close evolutionary relationship of sheep and cattle and the conserved synteny in these species of this extensive region of human Chr 4. On the basis of this conserved synteny, and the similar G- and Q-banding patterns of this chromosome in cattle and sheep, we propose that this sheep chromosome be numbered as 6, not 4 as recommended by ISCNDA (1990).

Characterization of Xenopus laevis complement factor I structure--conservation of modular structure except for an unusual insert not present in human factor I

Factor I (C3b/C4b inactivator) is a regulatory protein of the classical and alternative complement pathways. In this paper, we report the sequence of Xenopus factor I cDNA and the deduced protein structure. The basic structure of human preprofactor I, NH2-heavy chain-cleavage peptide-light chain-COOH, is conserved in the frog. However, the frog heavy chain contains a highly charged segment of 29 amino acids, encoded by a poly dA-rich mRNA insert, which is not found in human factor I. The modular structure of the frog heavy chain was analyzed, and found to differ vis-à-vis previously published analyses of human factor I. We also evaluate the timing of factor I transcription during frog embryogenesis.

The ovine Booroola fecundity gene (FecB) is linked to markers from a region of human chromosome 4q

The autosomal Booroola fecundity gene (FecB) mutation in sheep increases ovulation rate and litter size, with associated effects on ovarian physiology and hormone profiles. Analysis of segregation in twelve families (379 female progeny) identified linkage between the mutation, two microsatellite markers (OarAE101 and OarHH55, Zmax > 9.0) and epidermal growth factor (EGF) from human chromosome 4q25 (Zmax > 3.0). The marker OarAE101 was linked to secreted phosphoprotein 1 (SPP1, which maps to chromosome 4q21-23 in man) in the test pedigrees and independent families (Zmax > 9.7). The identification of linkage between the FecB mutation and markers from human chromosome 4q is an important step towards further understanding the control of ovulation rates in mammals.

Synteny mapping in the bovine: genes from human chromosome 4

Genes homologous to those located on human chromosome 4 (HSA4) were mapped in the bovine to determine regions of syntenic conservation among humans, mice, and cattle. Previous studies have shown that two homologs of genes on HSA4, PGM2 and PEPS, are located in bovine syntenic group U15 (chromosome 6). The homologous mouse genes, Pgm-1 and Pep-7, are on MMU5. Using a panel of bovine x hamster hybrid somatic cells, we have assigned homologs of 11 additional HSA4 loci to their respective bovine syntenic groups. D4S43, D4S10, QDPR, IGJ, ADH2, KIT, and IF were assigned to syntenic group U15. This syntenic arrangement is not conserved in the mouse, where D4s43, D4s10, Qdpr, and Igj are on MMU5 while Adh-2 is on MMU3. IL-2, FGB, FGG, and F11, which also reside on MMU3, were assigned to bovine syntenic group U23. These data suggest that breaks and/or fusions of ancestral chromosomes carrying these genes occurred at different places during the evolution of humans, cattle, and mice.

Factor H

While the mouse and human H proteins are structurally and functionally similar, they differ in their genetics. Whereas there is no evidence in humans for more than one gene; in mice the H locus is complex. Based on cDNA sequence and hybridization analysis of genomic cosmid clones, there are at least three distinct genes, all highly related to one another. The consensus repeating unit that comprises this molecule has obviously been duplicated numerous times, since it is present in many other molecules. Thus, it is not surprising to discover that there are several genes related to H in the mouse. A similar case has been described for two other members of this family. In humans, CR1 cDNA hybridizes to two distinct genomic clusters in the CR1 locus (Wong et al. 1989), and in mice, mCRY hybridizes to two regions in the genome, one on chromosome 1 and another on chromosome 8 (Aegerter-Shaw et al. 1987). It will be of interest to see if any other members of this family display as complex a genetic locus as murine H.

Mapping of the human complement factor I gene to 4q25

A detailed genetic and physical map of human complement factor I (IF) using somatic cell hybrids, in situ hybridization, and genetic linkage is reported. The gene has been localized to band 4q25. The order GC-INP10-ADH3-EGF-IF-IL2-MNS is proposed for genes on 4q on the basis of genetic and physical mapping techniques. A BclI polymorphism found with the IF probe demonstrated a maternal origin for a de novo deletion of chromosome 4 that was used in physically mapping the gene. The genetic and physical distances around band 4q24 suggest that 1 cM is approximately 1.2 million bp of DNA. This work provides a useful addition to the map of 4q.